This invention relates to formation of articles from propylene polymers which require a broad thermal processing window such as films and fibers and more particularly relates to oriented polypropylene ("OPP") films and fibers produced from a polymer made using a high activity catalyst and specific silane modifier compounds.
Some uses of propylene polymer resin require a larger thermal processing window than conventional resins for superior performance. Although recent advances in catalyst technology, such as use of magnesium halide-supported, titanium-containing catalyst components, have demonstrated increased activity and stereospecificity, the resulting product typically is more regular and crystalline than polymer products made from older catalyst systems. Such an increase in regularity and tacticity, produces a product with a sharper meting point and a narrow thermal processing window, Although many uses of propylene polymers do not require a broad processing window, some critical uses do benefit from such a broad window.
OPP films (sometimes referred to as biaxially oriented (BOPP) films) are used widely in packaging applications. In forming OPP films, propylene polymer resin is extruded and stretched in a machine (longitudinal) direction and simultaneously stretched in a transverse (horizontal) direction to form two axes of orientation. For optimal fabrication performance, the propylene polymer resin should have a specific combination of extractables and solubles components at a specific tacticity (measured by .sup.13 C nuclear magnetic resonance (NMR) technique) to produce a broad processing window. The ability to control the extractables/solubles and the tacticity of the polymer resin is of paramount importance in producing resins suitable for use in preparing OPP films.
First generation catalyst systems based on titanium tetrachloride and aluminum alkyl were very good at producing the specific polymer properties desired for OPP film resins. However, widespread use of magnesium halide-supported, titanium-containing high activity catalysts ("HAC") has resulted in changes to resulting polymer properties which are not advantageous for OPP film resins. These changes include a higher tacticity and lower extractables and solubles together with a blockier distribution of tacticity mistakes. The ability to control the extractables/solubles as a function of NMR tacticity relationship with a HAC catalyst would be very useful in producing improved OPP film resins.
Use of solid, transition metal-based, HAC, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide-based catalyst components.
Numerous individual processes or process steps have been disclosed to produce improved supported, magnesium-containing, titanium-containing, electron donor-containing olefin polymerization or copolymerization catalysts. For example, Arzoumanidis et al., U.S. Pat. No. 4,866,022, incorporated by reference herein, discloses a method for forming an advantageous alpha-olefin polymerization or copolymerization catalyst or catalyst component which involves a specific sequence of specific individual process steps such that the resulting catalyst or catalyst component has exceptionally high activity and stereospecificity combined with very good morphology.
In addition to the solid, magnesium-containing, titanium-containing HAC catalyst component, the polymerization catalyst system used to produce propylene polymers uses an aluminum alkyl component, such as triethylaluminum, and typically an external modifier component such as at silane compound as described in U.S. Pat. No. 4,829,038, incorporated by reference herein.
Use of external silane modifiers in a propylene polymerization catalyst system has been widely described. Use of alkyl or aryl methoxysilanes, and particularly dialkyldimethoxysilanes, has been described. The present invention describes using a specific silane composition as an external modifier to control the tacticity versus extractables/solubles relationship and thereby producing a resin which has a broad thermal processing window and is useful in producing a particularly beneficial OPP film. The family of such silanes comprises branched C.sub.4 -C.sub.5 alkyl methyl dimethoxysilanes and particularly isobutylmethyldimethoxysilane ("IBMDMS").
Use of IBMDMS as a modifier useful to produce propylene polymers has been described in U.S. Pat. No. 4,829,038 and Published European Application EP 0 361 371; however, neither of these references teach or suggest the specific properties of a propylene polymer actually produced using IBMDMS nor describe any possible advantage regarding the use of such modifier in producing an oriented polypropylene film.
U.S. Pat. No. 5,484,824 describes a thermoplastic resin composition useful in automotive bumpers comprising a crystalline polypropylene and an ethylene-butene copolymer rubber. IBMDMS is listed as a possible modifier useful in preparing the crystalline polypropylene, however, no specific properties of a polymer actually made with IBMDMS are described. Further, using such polymer to form a film or fiber is not suggested.
European Patent Application EP 0 657 476 is directed to an alpha-olefin polymer useful for OPP film which is made using various silane external modifiers such as n-alkylmethyldimethoxysilanes. Applicants find that the products of this invention are superior to those described in this reference in terms of an unexpected balance of product properties and catalyst yield.
European Patent Application EP 0 657 477 is directed to an alpha-olefin polymer useful for OPP film which is made using at least two silane external modifiers such as n-alkylmethyldimethoxysilanes.
As noted above, a superior HAC-produced propylene polymer with a broad processing window useful for manufacturing products, such as OPP films, has been desired. This invention describes a propylene polymeric resin which is useful in forming advantageous film products and especially OPP film.